Traditional web search engines retrieve a ranked list of URL's in response to a query from a user. Increasingly, search results include content from specialized sub-collections or corpora known as “verticals”, which may include non-text media collections such as images and videos, as well as genre-specific subsets of the web such as news and blogs. When a general web search engine has access to or maintains vertical search engines, one important task becomes the detection and presentation of relevant vertical results, known as “vertical selection”. An objective is to maximize the user satisfaction by presenting the appropriate vertical display or displays: this includes the presentation of no display when the user is best satisfied by general web results.
One important aspect of the task of generating a single ranked list from multiple sub-collections (“distributed information retrieval”) is deciding which sub-collections to search given a user's query. This may be approached using query classification techniques that automatically match queries to a predefined set of categories. This predefined set may be topical categories such as games, business, or health. However, this methodology is incomplete and does not take other factors into account.